Edit: Nonduality (advaita) is a metaphysical framework which originated in India. It is the view that the ground of reality is a single, unbounded, infinite, eternal consciousness, and all appearances of duality are illusion (maya). The two most well-known articulations of nonduality come from Advaita Vedanta and Kashmiri Shaivism.
What does the “!” do?
It creates an instance-agnostic link, so that whoever clicks it opens the community from their home instance. Opening it on your home instance is required to subscribe. A lot of the phone apps will do it automatically anyway, but it’s very valuable for those using web browsers.